Boeing examines expanded metal foils for lightning protection of composite structures

With the implementation of major aircraft structures fabricated from carbon-fiber-reinforced plastic (CFRP) materials, lightning protection has become a more complicated issue to solve. One widely used material for lightning strike protection of CFRP structures within the aerospace industry is expanded metal foil (EMF). EMF is currently used in both military and commercial passenger aircraft.

An issue that has historically been an area of concern with EMF is micro cracking of paint on the composite structure, which can result in corrosion of the metal foil and subsequent loss of conductivity. Researchers from Boeing Research and Technology (BR&T) examined the issues of stress and displacement in the composite structure layup, which contribute to paint cracking caused by aircraft thermal cycling (i.e., ground-to-air-to-ground flight cycle).

There are several contributors to the stress buildup, including the paint, primer, corrosion-isolation layer, surfacer, EMF, and the underlying composite substructure. BR&T focused primarily on the EMF contribution to the cracking mechanism, with computer-modeling analysis performed using commercially available COMSOL Multiphysics software that was supported by data from limited experimental testing.

The temperature cycle of the layers was simulated using a coefficient of thermal expansion (CTE) model developed with the software. The simulation allows determination of the thermal stress and displacements that result. Though the full complexity of crack genesis is not included, some insight can be gained regarding what the sensitive parameters of the EMF may be and the variations that can be employed to mitigate the resulting stress and displacements that lead to cracking.

Of particular interest are the EMF width, height, aspect ratio, composition—aluminum (Al) or copper (Cu)—and surface layup structure. In the case of Al used for EMF, fiberglass is needed between the aluminum and the structure to prevent galvanic corrosion.